Indium-tin-oxide particles are known for their unique electrical and optical properties, which depend on level of tin doping. For example, tin-doped indium oxide (indium-tin-oxide, ITO) thin films with optimum tin doping have high light transparency within the visible light spectrum and low electrical resistivity. Such ITO films have been used as electrodes in the manufacturing of solar cells, flat panel displays, heat shields and gas sensors. The doping level of tin in indium is a factor that is related to the optical and electrical properties of indium-tin-oxides. Conventional physical deposition techniques for the production of ITO films are DC sputtering, RF sputtering, or electron beam evaporation. Sputtering, for example, involves forming a film by sputtering at a sputter target and is used in many industrial applications especially glass coating and microelectronics.
Indium-tin-oxides are generally formed by mixing the respective oxides in a predetermined ratio, molding the mixture under dry or wet conditions and sintering the mold at the require temperature. Afterwards, the indium-tin-oxide is sputtered onto glass substrates by controlled electron beam heating. Such techniques require costly high vacuum equipment, and the utilization rate of the ITO material is low, as the materials are not selectively deposited on the substrate.
Metal oxide nanoparticles, synthesized from aqueous solutions for use in thin films of ITO deposited at low temperatures, are important for the preparation of transparent conductive films with high yield. Dip-coating or spray deposition of transparent ITO films with low resistivity, and low-membrane resistant ITO films are known processes which require nanomaterials of enhanced quality in terms of the size, level of tin doping and morphology of the nanoparticles.
With the development of nanometer material research, several methods for the production of nano-sized ITO particles have emerged. Known methods for nanoscale indium-tin-oxide preparation mainly include solid-phase methods. U.S. Pat. No. 7,601,661 purports to describe a solid phase process for producing an ITO powder at low cost and which provides high density sputtering targets having a longer lifetime. The approach involves creating a solid solution of indium and tin precursors, which can be a sputtered at a target. This approach, however, does not appear produce ITO nanoparticles or nanometer size ITO thin films that can be used directly. This approach appears only to enable formation of particles by evaporating the solid solution using magnetron sputtering. Furthermore, the ITO produced by this method may be of a low yield, yellow in colour and require further processing so as to dope the nanomaterial film.
Known liquid-phase methods of ITO production include liquid phase precipitation, hydrothermal (high temperature hydrolysis), Sol-gel (colloidal chemistry), and radiation chemical synthesis.
U.S. Pat. No. 5,401,441 purports to describe a method for the preparation of conductive metal oxide powders by forming a colloidal aqueous solution of crystalline particles of composite oxides of several metals by hydrolyzing a starting solution of metal ions and an agent for complexing the metals. An acid or base is added to the starting solution, and then heat-treated. The disclosed hydrolyzing process includes a process of adding acid or base to the starting aqueous solution causing a change of pH and formation of crystalline particles. As the pH changes, the concentration of reactants reduces and the composition of reactants shifts resulting in production of composite oxides having various ratios of the metals from one particle to another as they are produced throughout the reaction. The resulting final product may thus be a mixture of metal oxides having different compositions.
U.S. Pat. No. 6,533,966 purports to describe a co-precipitation ITO powder production method that involves, as described in the examples, the drop-wise addition of a base, such as hydroxylammonium, to a mixture of indium and tin compounds such as indium and tin chloride. During the addition of the base, such as for example hydroxylammonium, the pH of the reactant mixture changes from the ranges of 0 to 1 to the range of 3.5 to 4.5, depending on the exact reaction conditions, and then to 10 as an increasing ratio of indium to tin particles are formed. In addition, the reaction mixture, according to this method, is a mixture of unreacted indium and tin ions and precipitated materials. The concentration of unreacted ions reduces from 100% at the outset of the reaction to very low at the completion of the precipitation reaction. As such, the reacting ion concentration changes throughout the reaction. Thus, it is believed that the resultant indium-tin-oxide particles have various ratios of indium to tin and are not in substantially the same ratio as that found among the reactants at the outset of the reaction. In other words, it is believed that the reaction conditions are prone to drift through the course of the reaction and may result in particles with various ratios of indium to tin. This effect of reaction condition drifting is not disclosed. Furthermore, this method does not teach a substantially continuous process for the production of indium-tin-oxide under substantially constant reaction conditions where the reaction conditions may be adjusted so as to obtain a desired size range of ITO particles having a desired particle shape and a substantially consistent ratio of indium to tin in the formed particles of the resulting powder. Co-precipitation processes similar to those described in the abovementioned U.S. Pat. No. 6,533,966 are known where the co-precipitation is carried out under evolving pH conditions, by adding a high pH base to a very low pH mixture of indium and tin compounds. The pH changes during the precipitation would, it is believed, lead to products having particles with wide range of indium-to-tin ratios, and therefore to products having inconsistent electrical and optical properties, owing to operational efficiencies.
U.S. patent application publication serial number U.S. 2011/0036269 purports to describe a process for producing an ITO by adding a base, such as hydroxyl ammonium, to a mixture of indium and tin compounds. As a result, the pH of the reaction also changes gradually from a range of about 0 to 1 to a range of about 3.5 to 4.5 and then to 10 during reaction. This change of pH results in nanoparticle powders of indium and tin having different ratios of indium to tin since the indium and tin have different solubility constants at different pH levels. Furthermore, as the base, such as hydroxylammonium, is added to a vessel containing the mixture of indium and tin compounds in this production process, the ratio of indium to tin changes over time, owing to the gradual change in pH of the reaction. Therefore the ratio of indium to tin is not the same at various time points from the commencement of the precipitation reaction to the end, which may lead to an inconsistent ratio of indium to tin in the final the product.